Chemical modification of tannins to elaborate aromatic

Green Chemistry

CRITICAL REVIEW

Cite this: Green Chem., 2015, 17,2626

Received 6th February 2015,Accepted 26th February 2015

DOI: 10.1039/c5gc00282f

www.rsc.org/greenchem

Chemical modification of tannins to elaboratearomatic biobased macromolecular architectures

Alice Arbenz and Luc Avérous*

Tannins are, after lignins, the most abundant source of natural aromatic biomolecules and can be an alterna-

tive feedstock for the elaboration of chemicals, building blocks to develop polymers and materials. Tannins

are present in all vascular and some non-vascular plants. One of their major issues is the versatility according

to their botanical origin, extraction and purification processes. During the last few decades, tannins have

been exploited and chemically modified for the development of new biobased polymers, thanks to their

functionality brought by phenolic and aliphatic hydroxyl groups. After a historical overview, this review sum-

marizes the different classes of tannins. Some generalities concerning the extraction techniques of tannins

and the corresponding properties are also described. This review provides in detail the different chemical

modifications of tannins which have been previously reported, with corresponding pathways and appli-

cations. Finally, the main chemical pathways to obtain polymeric materials are especially presented.

1. Introduction

After cellulose, hemicellulose, and lignin, tannins are the mostabundant compounds extracted from the biomass. Tannins

are, after lignins, a major source of polyphenolic componentswith 160 000 tons potentially biosynthesized each year all overthe world.1 Tannins are found in various proportions in all vas-cular plants and in some non-vascular plants, such as algae.Their role in vascular plants is defense against mushroomor insect aggression.2 In addition, the natural astringency oftannins makes the plant not easily digestible by animals.3

In non-vascular plants, phlorotannins are oligomers ofphloroglucinol (1,3,5-trihydroxybenzene) and are found mostly

Alice Arbenz

Alice Arbenz received herMaster’s degree in chemistrywith specialization in materialsand polymers in 2011 at theEngineering School for Chem-istry in Mulhouse, France. Sheearned a Ph.D. in materials andpolymer chemistry in 2015 withProf. L. Avérous at ICPEES/BioTeam (Institute of Chemistryand Processes for Energy,Environment and Health – Uni-versity of Strasbourg/CNRS) inStrasbourg, France. Her interests

cover several aspects of biobased chemicals and renewablepolymer materials. During her Ph.D. thesis, her focus was on thesynthesis of new biobased building blocks (tannin derivatives) inorder to develop controlled aromatic macromoleculararchitectures.

Luc Avérous

Luc Avérous became in 2003 aFull Professor at ECPM (Univer-sity of Strasbourg – France),where he teaches biopolymerscience, biomaterials, compo-sites, plastic processing &polymer characterization. Duringthe last two decades, hisresearch projects have dealt with“biobased and/or biodegradablepolymers systems, for environ-mental and biomedical appli-cations”. As a leadinginternational expert in these

fields, he has developed strong collaborations with several foreignlabs (Australia, Brazil, Canada, and Spain) and major companies(PSA, Soprema, Tereos, and Total). He has published hundreds ofscientific communications and has co-edited 3 books during thelast 5 years.

BioTeam/ICPEES-ECPM, UMR CNRS 7515, Université de Strasbourg, 25 rue

Becquerel, 67087 Strasbourg, Cedex 2, France. E-mail: [email protected];

Fax: +333 68852716; Tel: +333 68852784

2626 | Green Chem., 2015, 17, 2626–2646 This journal is © The Royal Society of Chemistry 2015

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in brown algae (2% of dry mass)4,5 and in some red algae.6

Their chemical structures are similar to those of vascular planttannins.7 Phlorotannins have different metabolic roles,8 suchas cell wall construction,9 marine herbivore defense e.g.against worms,10 and UV protection.11

Tannins are present in each cytoplasm of all vegetablecells.12 Contrary to lignins, tannins are mainly in soft tissuesof the plant such as sheets, needles or bark.13 According to thebotanical resource, tannins are stored in different zones of theplants:12 as the bark in pine (Pinus sp.), oak (Quercus sp.) ormimosa (Acacia mearnsii), the sheets in gambier (Uncariagambier), or the wood in quebracho (Schinopsis sp.) and chest-nut (Castanea sp.). Some tannins are very well known in every-day life, such as tannins from tea and wine.14–17

2. Tannin chemistry2.1. Historical outline

The term “tannin” indicates a plant material which allows thetransformation of hide into leather.12 The history of tanninshas mainly begun between 1790 and 1800.18 At that time, therewas no differentiation between tannin and gallic acid.19 In 1787Scheele discovered gallic acid from oak bark. Dizé continuedresearch on galls in 1791 and Deyeux in 1793. They agreed thatthe isolated substance was a mixture of gallic acid, a green col-oring compound, and other molecules. In 1795, Seguin didtanning experiments with oak-bark and developed the tanningprinciple.20 He concluded that when he boiled hide water withan oak-bark infusion, he obtained a light-colored precipitate.The latter was insoluble in water and became dark after lightexposure. Thereafter, Berzelius prepared almost pure tannin in1798. He used a decoction of galls which formed a yellow pre-cipitate after the addition of tin dichloride. The precipitate wasdecomposed in water with sulphuric acid, forming metalsulfide, which precipitated, leaving the astringent componentin solution. Finally in 1834, Pelouze observed the formation ofcrystalline gallic acid from tannin after being boiled with sul-phuric acid. In 1891, Trimble defined “tannin” as the wholeclass of astringent substances. Nowadays, the common defi-nition of tannin is the one given in 1962 by Swain and Bate-Smith,21 i.e., tannins are water-soluble phenolic compoundswith molecular weights between 500 and 3000 g mol−1.

Tannin structures were studied in order to find their chemi-cal organizations. The most difficult step was to obtain a puresubstance, free from sugars. The early purified substances cor-responded to digallic acid. In 1912, Fischer and Freudenbergshowed that the tannin structure was pentadigalloyl glucose,which is the pentahydroxy gallic acid ester of glucose.22 Now-adays, investigations are being carried out to elucidate thechemical structures of different tannins.23–36

2.2. Classification and chemical structures of vascularplant tannins

The word “tannin” is used to define two classes of phenoliccompounds with different chemical natures: hydrolysable

tannins and condensed tannins.37 Hydrolysable tannins aresubdivided into gallotannins and ellagitannins. Another classcan also be considered: the complex tannins.38 All thesedifferent classes are presented in Fig. 1.

2.2.1. Hydrolysable tannins. Hydrolysable tannins are amixture of phenols such as ellagic acid and gallic acid, estersof sugars (i.e., glucose), gallic acid or digallic acid.39 Thesetannins are divided into two families: the gallotannins, whichproduce gallic acid and its derivatives from hydrolysis; and theellagitannins, which produce ellagic acid after hydrolysis(Table 1). These tannins are mainly used in the tanning indus-try. The most used are tannins of chestnut and tara. Theypresent a weak nucleophilicity. They also present low avail-ability with limited worldwide production (less than 10% ofthe world’s tannin commercial production) and a relativelyhigh price which makes them less attractive compared to con-densed tannins.40

2.2.1.1. Gallotannins. Gallotannins are simple hydrolysabletannins resulting from polyphenolic and polyol residues.Although the nature of the polyol residues can vary, most gallo-tannins are attached to saccharides. The hydroxyl functions (OH)of this polyol residue can be partly or completely substituted bygalloyl units. If this substitution is partial, the remaining OHcan be substituted or not by other residues. For example, inTable 1, the gallotannins 2,3,4,6-tetra-O-galloyl-D-glucopyranose(TGG) and 1,2,3,4,6-penta-O-galloyl-β-D-glucopyranose (β-PGG),which are present in many plants, are key intermediaries in thebiosynthesis of the hydrolysable polyphenols.41

2.2.1.2. Ellagitannins. Ellagitannins constitute the mostknown class of tannins with more than 500 identified naturalproducts.42 They are formed by gallotannins which lead to amonomeric unit with an axis of chirality after an oxidativecoupling between at least two galloyl units. The basicmonomer is the hexahydroxydiphenol (HHDP) presented inTable 1. The observed chirality is caused by the presence ofbulky substituents located in the ortho positions of the biarylaxis and by the isomerism caused by the restricted rotationaround this axis. Very often, esterification in ortho of the twocarboxyl groups with a polyol (generally D-glucopyranose) is atthe source of this chirality.

2.2.2. Complex tannins. Complex tannins are formed froman ellagitannin unit and a flavan-3-ol unit.43 An example of

Fig. 1 Tannin classification.

Green Chemistry Critical Review

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this kind of tannin is acutissimin A (Table 1), which is com-posed of a flavagallonyl unit connected to a polyol derivedfrom D-glucose by a glucosidic connection in C-1 and threeother ester bonds.44

2.2.3. Condensed tannins. Condensed tannins representmore than 90% of the worldwide production of commercialtannins.37 From 3 to 8 flavanoid repetition units are needed to

call a compound condensed tannin. These tannins are alsomade up of associated precursors (flavan-3-ol, flavan-3,4-diol)of carbohydrates as well as amino and imino acid traces.45

These tannins are generally complexed with proteins.46,47

Their faculty of complexation changes according to theirchemical nature.48 Each flavanoid is composed of two phenolicrings having different reactivities.12

Table 1 Structure of vascular plant tannins

Type of tannins Examples of structures

Hydrolysable tannins Gallotannins

Ellagitannins

Complex tannins

Condensed tannins

Critical Review Green Chemistry


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The structure of monoflavanoid in Table 1 shows that it ispossible to have two configurations for each ring: with orwithout OH in positions 5 and 5′. These various configurationsgenerate four possibilities (Fig. 2) and four correspondingbasic building blocks from condensed tannins.49

The flavanoids from condensed tannins are mostly derivedfrom flavan-3-ol and flavan-3,4-diol.50 The type of connectionbetween the various flavanoid units depends on the nature ofthe rings. A resorcinol type A-ring (substitution by an OH inthe meta position) of the flavan-3,4-diol as well as the oxygenin the heterocycle create high nucleophilicity on C6 and C8positions. Thus, the units of condensed tannins are mainlyconnected through C4–C6 or C4–C8 bonds. C4–C6 is prevalentin tannins mainly composed of profisetidins and prorobineti-dins. C4–C8 is prevalent in tannins mainly composed of pro-cyanidins and prodelphinidins.51

In a flavanoid, nucleophilic centers of the A-ring are gener-ally more reactive than those of the B-ring. This is due to theposition of the OH present on the rings, which are at theorigin of this difference in reactivity. In the case of the A-ring,they involve a located activation (C6 and C8) due to the reson-ance structure, leading to a highly reactive nucleophilic centercontrary to the case of the B-ring.39

2.3. Non-vascular plant tannins

Phlorotannins are present in non-vascular plant tannins. Theyare formed by the polymerization of phloroglucinol (1,3,5-tri-hydroxybenzene) and have a large range of molar massesbetween 126 and 650 000 g mol−1.4 However, the molar massgenerally varies between 10 000 and 100 000 g mol−1.52 Phloro-tannins can be easily compared to condensed tannins fromvascular plants. Their roles are relatively similar as well astheir chemical reactivity. For example, phlorotannins can beassociated with metal ions or various biomacromolecules suchas proteins, glucides or nucleic acids. However, phlorotanninsare not produced by the same biosynthesis method used for

other conventional tannins. That is because phloroglucinolderives from the condensation of acetate and malonate via anenzymatic reaction.

Phlorotannins can be subdivided into four classes accord-ing to the type of linkage between the phloroglucinol units(Fig. 3). There can be either ether bonds (fuhalols and phlor-ethols), phenyl bonds (fucols) or both (fucophlorethols).Finally, the last type of linkage is a dibenzo-p-dioxin connec-tion (eckols and carmalols).6

2.3.1. Phlorotannins with ether bonds. This category iscomposed of fuhalols and phlorethols, which both derive fromphloroglucinol units connected together by aryl–ether bonds.The corresponding molecules can be linear or not. In the caseof the phlorethols, compounds are strictly made of phloro-glucinol units. In the case of the fuhalols, the phloroglucinolunits are connected by ether bonds having regular sequencesin ortho and para positions and containing in all the rings anadditional OH (Table 2).53

2.3.2. Phlorotannins with phenyl bonds. This class ismade of compounds formed by phloroglucinol units which areonly connected by aryl–aryl bonds. This group is essentiallycomposed of fucols (Table 2).

2.3.3. Phlorotannins with ether and phenyl bonds. Somecompounds are made of phloroglucinol units which are con-nected by ether–aryl and aryl–aryl bonds. These molecules arecalled fucophlorethols (Table 2).

2.3.4. Phlorotannins with dibenzo-p-dioxin links. Thisclass is made up of two types of molecules. The eckols aremade of at least half a molecule of dibenzo-p-dioxin substi-tuted by phloroglucinol in the C4 position. Carmalols are com-posed of one half of a dibenzo-p-dioxin molecule andphlorethol derivatives (Table 2).

3. Extraction of tannins

Plant preparation and extraction processes have a great influenceon tannin extract composition. Phenolic compounds can beextracted from fresh, frozen or dried plant. Generally, after dryingtreatment, plants can be milled and homogenized.54,55 Dryingmethods have a strong impact on the final composition of thetannin extracts, with e.g. bonding with other compounds.56–58

Fig. 2 Structures of the four monoflavanoid building blocks.

Fig. 3 Phlorotannin classification.



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Under anaerobic conditions, the free tannin content increasesslightly with the temperature, whereas under aerobic conditions,there is a strong decrease in the free tannin content. Lyophilizedsamples are similar to those dried at a lower temperature (25 and45 °C). Compared with other drying methods, freeze-dryingseems to be one of the best methods for preserving the molarmasses of the condensed tannins,59 extracting high levels of phe-nolic compounds60 and preserving the native structures.61

Extraction methods are usually based on solvents. Theindustrial method of extraction traditionally used to recovertannins starting from vegetable substances is based on boiling

water. The corresponding solution is then concentrated byevaporation.62 Wood shavings are loaded in a series of auto-claves functioning by counter-current. This system of auto-claves is used in order to solubilize tannins contained in astrong concentration in fresh wood shavings. This treatment,called scrubbing, is generally carried out with water at a fixedtemperature (50–110 °C) and pressure (maximum 0.8 bar) ineach autoclave for several hours (6–10 h) for a ratio of water/wood equal to 2–2.4 in mass. Generally, this process leads to asolution containing 4–5% in weight of tannins with an extrac-tion yield of around 60–65%. After clearing by decantation, the

Table 2 Structure of phlorotannins

Types of phlorotannins Examples of structures

Ether bonds

Phenyl bonds

Ether and phenyl bonds

Dibenzo-p-dioxin links



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tannin solution is concentrated by several evaporations undervacuum, to limit the oxidation of tannins, until obtaining thedesired concentration (in general, 40–50% in weight). This lastsolution can be stored after addition of a stabilizing agent or itcan undergo other treatments like being reduced to drypowder by atomization, i.e. by spraying with hot air (90–96% ofdry mass). Water can be used alone, but most of the timeorganic solvents with or without water such as ethanol,63–65

methanol,66–68 acetone,57,69–72 ethyl acetate73 or mixtures ofthese solvents74 are used to increase the extraction efficiency.75

To improve the extractive yield, alkaline solutions are alsoused. The most studied species is pine wood.76–82 Effects ofalkaline solution extraction are also the same on chestnut andeucalyptus species83 or on cranberry pomace.84 Another tech-nique for increasing the yield is by the use of an acidic solvent.Strong acids like hydrochloric acid or a weak acid like formicacid can be used.70,75,85,86 However, these extraction methodsneed large volumes of organic solvents as well as long extrac-tion times. That is why new methods have been developed overthe past few years such as microwave-assisted extraction(MAE),87–91 ultrasound-assisted extraction (UAE)92–95 and pressur-ized liquid extraction (PLE), also called accelerated solvent extrac-tion (ASE)96,97 with water (subcritical water extraction, SWE)98,99

or solvent (supercritical fluid extraction, SFE).100,101 These newmethods bring strong improvements in tannin extraction.

The chemicals used depend on plant parts102,103 or species25,64

with a variety of physical properties like polarity or solubility.104

There are no universal extraction conditions. Each plant samplehas an optimized solvent and an extraction procedure.105 Theyield and the composition of extracts106,107 rely totally on the typeof solvent (polarities),108–110 the extraction time and the tempera-ture of the process98,111,112 and the sample/solvent ratio.68,110

Most of the time, tannins are bound with other plant com-ponents such as carbohydrates or proteins, and during extrac-tion, various plant components can be also extracted such assugar or organic acids. Currently, these impurities are thebiggest hindrance limiting a global and successful developmentof tannin as the biobased resource to develop aromatic buildingblocks. To obtain “pure” tannins, additional and costly treat-ments are necessary to purify the extracted mixture like liquid–liquid extraction procedures or solid phase extractions.54

4. Chemical modification4.1. General background

This section will be mainly focused on the chemical modifi-cation of condensed tannins. A wide variety of modifications,linked with their chemical structure, can be performed withthese tannins. As presented in Fig. 4, the main chemical

Fig. 4 Various types of reactions with catechin, applicable to condensed tannins.



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modification can be classified into three main categories. Theheterocycle can be opened and can lead to rearrangements ofthe chemical structure. Reactivity of nucleophilic sites, createdby OH groups present on the aromatic rings, leads to electro-philic aromatic substitutions. Lastly, reactions can also takeplace directly with the OH. In each category, there are differentreactions leading to new building blocks, which are of greatinterest for polymer synthesis.

4.2. Heterocycle reactivity

Under both acidic and alkaline conditions, catalyzedrearrangements such as hydrolysis and autocondensation arecommon reactions for tannins (Fig. 5). The cleavage of theinterflavonoid bond can also be acid-catalyzed or induced by asulfonation reaction.

4.2.1. Hydrolysis and autocondensation. Under strongacidic conditions, hydrolysis or autocondensation can occur.Degradation under acidic media leads to the formation of cate-

chins and anthocyanidins. The example of biflavanoid is illus-trated in Fig. 6.113

The second type of reaction is the condensation after hetero-cycle hydrolysis (Fig. 7). The formed p-hydroxybenzylcarbo-nium ions condense on the nucleophilic sites of anothertannin unit. Phlobaphens or red tannins are obtained.

In the same way, under alkaline conditions differentrearrangements are common. These rearrangements are basedon the rupture of the interflavonoid C4–C8 bond. The reactivitydepends on the nature of tannins. The formed products canautocondense, giving an alkaline condensation (Fig. 8).114

The second type of reaction is based on the opening of theheterocycle which increases the reactivity, resulting in partialautocondensation, as for catechin monomers (Fig. 9).115

4.2.2. Heterocyclic ring opening with an acid. The cleavageof the interflavonoid bonds can be acid-catalyzed. This leads

Fig. 6 Degradation of tannins in catechins and anthocyanidins.

Fig. 5 Summary of the chemical reactions from tannin heterocycles. Fig. 7 Acid autocondensation by hydrolysis of the heterocycles.



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to a heterocyclic ring opening with the formation of a carbo-cation (Fig. 10), which can be captured by a nucleophile suchas phenol, resorcinol, phloroglucinol or the phenolic rings ofother flavonoid units present. Different acids were tested suchas hydrochloric acid116 or acetic acid catalyzed with benzylmercaptan.117,118

To open the heterocycle, it is also possible to treat tanninswith trichloroacetic acid,119,120 which is added in an aqueoussolution of tannins, and then heated at 86 °C for 2 hours.After cooling, the reaction medium is neutralized by theaddition of a sodium hydroxide (NaOH) solution.

4.2.3. Sulfonation. It is also possible to open the hetero-cycle by a sulfonation reaction.121–123 The reaction takes placein the presence of sodium hydrogen sulfite or sodium hydroge-nosulfate with the insertion of a sulphonic group into position2 after the heterocyclic ring opening (Fig. 11). This polar groupincreases the solubility and decreases the viscosity of tannins.

This reaction is often used to facilitate the tannin extrac-tion. Sulfited tannins can be used to produce formaldehyderesins.124 However, sulfated tannins are moisture sensitive.When they are submitted to strongly alkaline conditions, thesulphonic group can be substituted by an OH in the presenceof strong alkali via a bimolecular nucleophilic substitution(SN2) reaction (Fig. 12).

4.3. Reactivity of nucleophilic sites

Nucleophilic sites are present in the tannin structure due tothe phenol groups. Indeed, electrophilic aromatic substi-tutions can occur. In fact, OH is an electron donor which canbe intensified under basic conditions in which a phenoxideion is formed. Bromination reaction was studied to under-stand this reactivity. Then, different modifications were investi-gated to tune the tannin structure. Modifications wereperformed with hexamine, furfuryl alcohol, ethanolamine anddifferent aldehydes. Due to the aromatic structure, the coup-ling reaction can also occur with carboxylic acid and alde-hydes. All these modifications are presented in Fig. 13.

4.3.1. Bromination. To study the reactivity as well as theaccessibility of the flavanoids, some model molecules resultingfrom phloroglucinol and resorcinol have been selectively bro-minated in pyridine.49 Phenol groups are ortho and paradirecting. Thus, on the A-ring, C6 and C8 are both activated bythe phenol groups. In the case of (+)-tetra-O-methylcatechin,bromination is done preferentially in the C8 position. Whenthis site is occupied, substitution is performed in the C6 posi-tion since the B-ring is less reactive. In the case of an excess of

Fig. 8 Alkaline autocondensation.

Fig. 9 Catechin rearrangement.

Fig. 10 Treatment of mimosa tannin with acid.

Fig. 11 Sulfonation reaction of tannin.

Fig. 12 Substitution of the sulphonic group by OH.



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the bromination reagent, the reaction can also take place inthe C6′ position. Thus, the sequence of bromination is the fol-lowing: C8, C6, and then C6′. However, for equivalent resorci-nol forms, such as (−)-tri-O-methylfustin, the brominationsequence is different with C6, C8 and finally C6′. Preferentialsites are presented in Fig. 14 depending on the flavonoids.

This preferential substitution in C8 vs. C6 is certainlyrelated to the accessibility of the sites for each type of flava-noid (phloroglucinol and resorcinol, respectively).

4.3.2. Reactions with aldehydes. In the case of adhesiveand foam preparation, formaldehyde is the most commonlyused aldehyde (see sections 5.1 and 5.2). With condensedtannins, formaldehyde reacts mainly with the A-ring to formmethylene interlinks. When the A-ring is of the resorcinoltype, the reactive site is in the C8 position, whereas, when theA-ring is of the phloroglucinol type, the reactive site is in theC6 position (Fig. 15).121 When alkaline conditions are used (atpH = 8), the nucleophilicity of tannin phenols is activated withthe formation of phenoxide ions.

The B-ring (pyrogallol or catechol) is less reactive comparedto the A-ring. Indeed, activation due to the presence of phenolgroups on the B-ring is not located on a carbon contrary to theA-ring whose activated sites are on C6 and C8. However, athigh pH (approximately 10), the B-ring can be activated byanion formation. Different tannin varieties and wastewaterfrom the tanning process have been successfullytested.78,124–129 Thus, it is possible to carry out this reactionregardless of the type of tannin.

The reaction between the A-ring and the formaldehyde(Fig. 16) leads to the creation of a cross-linked three-dimen-sional network with a decrease of the molecule mobility. Witha small degree of condensation, the size and the configurationof the molecules do not allow the creation of extra methylenebonds because the reactive sites are too far apart.

Fig. 13 Summary of the chemical reactivity of tannin nucleophilic sites.

Fig. 14 Bromination reactive sites of flavonoids.

Fig. 15 Formaldehyde reactive sites.

Fig. 16 Reaction between tannin and formaldehyde.



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The kinetic differences between formaldehyde and otheraldehydes were studied on tannins of pine and mimosa.125

Formaldehyde exhibits the fastest kinetics compared to otheraldehydes (acetaldehyde, propionaldehyde, iso-butyraldehyde,n-butyraldehyde or furfural) due to their steric hindrance.

4.3.3. Reaction with hexamine. To avoid the toxic form-aldehyde, hexamethylenetetramine, also called hexamine,was studied as a substitute. When under acidic conditions,hexamine is decomposed into formaldehyde and ammonia130

whereas under alkaline conditions, it decomposes into form-aldehyde and triethylamine.131 However, under certain con-ditions, it is possible to avoid the production of formaldehydefrom hexamine. When molecules with nucleophilic sites suchas tannins are present in the medium, amino-imine groupsare formed and react with the phenolic compounds (Fig. 17).The very fast reaction between amino-methylene bases andtannins prevents the formation of formaldehyde.132 This reac-tion leads to the formation of benzylamine bonds on thetannin molecules.

The decomposition of the hexamine strongly depends on pHwhich also has an influence on tannin reactivity. A study wasalso performed on autocondensation between hexamine andmimosa (condensed tannins) or chestnut tannins (hydrolysabletannins). It was shown that pH has a great influence on thereactivity.133 At high pH, hexamine decomposition is faster thanits reaction with tannin. In this case, it is possible to form for-maldehyde due to the decomposition of amino-methylene.

4.3.4. Reaction with furfuryl alcohol. Tannin can reactwith furfuryl alcohol to give intermediates for further synthesiswith e.g. formaldehyde. Furfuryl alcohol is a bio-sourcedheterocyclic alcohol derived from hemicellulose. The reactionwas studied on catechin and was carried out in the presence ofacetic acid at 100 °C. The acidity of the reactive medium isnecessary because at low pH, furfuryl alcohol reacts mainlywith itself.134 After purification, two products were obtained.Catechin is substituted by the furanyl group in the C8 positionand in the C6 position with a yield of 4 and 1.5%, respectively(Fig. 18).

However, these reactions show low yields. This is due to theself-condensation of furfuryl alcohol under acidic conditions,leading to poly(furfuryl alcohol).135

Some studies used formaldehyde and furfuryl alcohol toimprove the alcohol reactivity.136 Indeed, furfuryl alcohol canbe converted into 2,5-bis(hydroxymethyl)furan by formylation.

The latter is commonly used, mixed with tannin as a reactivespecies, to produce foams.

4.3.5. Mannich reaction. Under strong acidic conditions,tannins can react with ethanolamines according to theMannich reaction (Fig. 19), and then give amphoteric tannins,which are water soluble.121 These new compounds have floccu-lating properties and are used in water treatment plants toeliminate clay suspensions. After a Mannich reaction and analkylation, the quebracho tannins have flocculating propertiesnecessary to bleach waste waters.137

To understand the reactivity of this system, a study was per-formed on quercetin with various secondary amines such asdiethylamine or piperidin. Depending on the ratio of amine–formaldehyde used, it is possible to obtain mono- or di-(amino-methyl) quercetin (yield: 45% and 46%, respectively).138

4.3.6. Coupling reaction4.3.6.1 Michael reaction. Phenylpropanoids can be grafted

on the A-ring of catechin (Fig. 20). This synthesis is based on atwo-step pathway. A dienone-phenol rearrangement is followedby a Michael reaction type coupling.139

The reaction is catalyzed by trifluoroacetic acid (TFA) andsodium acetate (NaOAc). The best yields were obtained for amixture of tetrahydrofuran (THF)–benzene 1 : 1 (v/v). Othercompounds were tested such as cinnamic acid or p-methoxy-cinnamic acid but the coupling reaction did not occur. Thiswas due to the absence of the p-hydroxyl group in these mole-cules, which would have provided the formation of the inter-mediate through dienone-phenol rearrangement (tautomerism).This intermediate would then react with flavan-3-ol as aMichael acceptor.

4.3.6.2 Oxa-Pictet–Spengler reaction. This reaction isderived from the Pictet and Spengler reaction which consistsof β-arylethylamine cyclization after condensation with analdehyde in the presence of an acidic catalyst. In the case of

Fig. 17 Decomposition under alkaline conditions of the hexamine in m-ethylene imine–amine, which reacts with tannins.

Fig. 18 Products of the reaction between catechin and furfuryl alcohol.

Fig. 19 Mannich reaction with tannins.



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the reaction Oxa-Pictet–Spengler, the nitrogen is replaced byan oxygen atom (Fig. 21). This reaction was done with differentketones and catechins.140,141

The conventional catalyst is boron trifluoride etherate(BF3·O(Et)2). Catechin, ketone and a catalyst are mixed for 48 hat room temperature under a nitrogen atmosphere to formβ-arylethylamine cyclization.

4.4. Functionalization of the hydroxyl groups

Phenolic and aliphatic hydroxyl (OH) groups are reactive func-tions which can be modified to obtain several tannin deriva-tives. Tannin can be modified to increase the OH chemicalreactivity or its solubility in organic solvents, or to improve itsprocessing. Two main strategies are possible to obtain newbuilding blocks from tannin: (i) changing the nature of thereactive sites or (ii) increasing the hydroxyl group reactivity.The phenolic hydroxyl groups are the most active groups.Different modifications such as acylation, etherification, sub-stitution by ammonia and reactions with epoxy groups or withisocyanates were performed and investigated (Fig. 22). Thereactivity and the availability of phenolic functions can beincreased by modification into epoxy, which can be then trans-formed into aliphatic hydroxyl groups.

4.4.1. Acylation. Acylation of tannins is a well-knownmethod usually employed for the tannin characterization.After reaction with acetic anhydride in pyridine, tanninsbecome soluble in organic solvents142 and then can be ana-

lyzed e.g. by NMR.143 All OH (phenolic and aliphatic) are sub-jected to acylation.144 Whatever the level of acylation, thederivative of tannin loses its water solubility and melts ataround 150 °C.

Acylation agents are either carboxylic acids or more reactivederivatives, such as acyl chlorides or acid anhydrides.145 Esterscan also be used for transesterification reactions. If an acid isused, it is possible to form the corresponding anhydride insitu in the presence of N,N′-dicyclohexylcarbodiimide. Solventsgenerally used are toluene, pyridine, chloroform or acetone tosolubilize, at least partially, the polyphenolic compound. Thereaction was carried out in the presence of a base in order toactivate the hydroxyl groups. Bases were selected from organic

Fig. 21 Oxa-Pictet–Spengler reaction of catechin and various ketones.

Fig. 22 Summary of the chemical reaction with tannin OH groups.

Fig. 20 Dienone-phenol rearrangement and reaction between catechinand caffeic acid.



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or inorganic bases, such as pyridine, or potassium carbonate,respectively, when another solvent was used. The reaction pre-ferentially takes place at solvent reflux temperature until fullesterification. This reaction is considered as an opportunity toenhance the lipophilicity of tannins. For example, Fig. 23 pre-sents the reaction between quercetin and lauroyl acid chloride,in pyridine at 100 °C for 6 hours.

A complete study was performed on quercetin acetyl-ation.146 In most cases, the reaction is carried out with acylchlorides in 1,4-dioxane at 60 °C.

Tests were carried out directly on pine and quebrachotannins. In these cases, acylations were performed with stear-oyl chloride and 1-methylimidazole as a catalyst in acetone.143

The maximum degree of substitution increased with thestearoyl chloride/tannin ratio, for quebracho and pine tannins.The corresponding substitution degrees are 4.0 and 5.1,respectively.

As previously mentioned, tannins can be modified withfatty acid. Quebracho and pine tannins were esterified withlauroyl chloride to give the corresponding laureate ester.147

Partially substituted derivatives have been obtained with a pre-ference for substitution on the B-ring, with the formation ofmono- and di-esters. The same tannins have also been esteri-fied with acid, linoleate and acetate groups to produce tannin-fatty esters.148 Linoleic acid is firstly modified to linoleic acidchloride which reacts with the tannins.

Pine tannin esterification using anhydrides (acetic, propio-nic, butyric or hexanoic anhydride) was investigated as a routeto synthesize tannin esters possessing varying ester chainlength and degree of substitution.149 These tannin derivativeswere used as plastic additives in e.g. poly(lactic acid) asanti-UV.

Tannins can be regioselectively benzoylated according tothe Schotten–Baumann method (Fig. 24)121 to decrease thenumber of OH per flavonoid unit. Besides, they precipitate inaqueous solution to easily separate sugars from tannins.

Adipoyl dichloride is used to introduce a carbonaceouschain between tannin molecules. The highest yield (around75%) is obtained for molar ratios adipoyl dichloride : tanninequal to (6–10) : 1. The reaction is carried out at 75 °C for120 min in a mixture of chloroform and 1,4-dioxane.

Various carboxylic acids were grafted (Fig. 25) on the OH inposition 7 of the 5,7-dihydroxy-6-methoxy-2-phenylchromen-4-one (oroxyline A).150 A solution containing a carboxylic acid,

1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride(EDAC) and hydroxybenzotriazole (HOBt) in dichloromethaneis maintained at 0 °C for 20 min under a nitrogen atmosphere.Oroxyline A, solubilized in dichloromethane (DCM) and di-methylformamide (DMF), is then added. The reaction mediumis then stabilized at room temperature for 4–5 hours under anitrogen atmosphere.

Recently, the synthesis of urethane groups from non-iso-cyanate reactions was performed from hydrolysable chestnuttannins. By an aminolysis way, condensed tannins reactedwith dimethyl carbonate and then with hexamethylenediamine(Fig. 26).151,152 These promising pathways bring new routes tosafely synthetize high performance polyurethane materials,without toxic isocyanates.

4.4.2. Etherification. The polyphenols can react by etherifi-cation with halogenoalkanes. Alkylation of quercetin in DMFin the presence of tetraethylammonium fluoride (Et4NF) gives

Fig. 23 Acylation of quercetin by lauroyl chloride.

Fig. 24 Benzoylation of tannin of mimosa according to the Schotten–Baumann method.

Fig. 25 Synthesis of oroxyline A derivatives.

Fig. 26 Synthesis of urethane groups from non-isocyanate reactionswith condensed tannins.



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tetra-o-ethyl-3,7,3′,4′-O-ethyl[3H]-5-quercetin (Fig. 27).153 Thesame product can be obtained by reaction of quercetin withethylene carbonate. Depending on the quercetin/ethylene car-bonate ratio, synthesized products were more or lesssubstituted.154

Quercetin was also alkylated with 1-chloro-2-propanol inthe presence of potassium carbonate in DMF. Then, varioushydroxypropylquercetin could be obtained.155 It is also poss-ible to methylate OH from the rings. The reaction was studiedon chrysin with iodomethane (MeI) and K2CO3, as presentedin Fig. 28.156

Under the same conditions, this reaction is often used forspecific methylation of certain OH after the protection of someothers. For example, in the case of the catechin, after protec-tion of B-ring hydroxyls, it is possible to methylate catecholwith dichlorodiphenylmethane.157 When the targeted OH is m-ethylated, deprotection can be performed. The reaction wascarried out on quercetin to obtain 7-O-methyl quercetin(Fig. 29).158

Other experiments were completed with a mixture of halo-genoalkanes (MeI and ethyl bromide) on oroxylin A in the pres-ence of K2CO3 in acetone under reflux to develop newantibacterial agents.150 However, many fastidious steps arenecessary to produce the corresponding targeted molecule.Then, this pathway is not really industrially viable.

Investigations were directly performed on tannins. Pine andquebracho tannins were carboxymethylated by chloroaceticacid or chloroacetate sodium. A base is added into the reactionmixture: either NaOH if the reaction takes place in water, orK2CO3 if the reaction takes place in acetone. The best resultsare obtained when chloroacetic acid is used in acetone withK2CO3 (Fig. 30).

159

In addition, tannins react also with the 1-bromooctane inDMF with potassium carbonate at 60 °C. The OH reactinggroup percentage varies between 15 and 93%. These etherifiedtannins are insoluble in water.159

4.4.3. Substitution by ammonia. Amination reactions withammonia (NH3) were studied. It is possible to convert a part ofphenolic OH into amine functions. A study showed that amin-

ation of the pyrogallol B-ring by an ammonia solution, withouta catalyst and under mild conditions, is regioselective.160 Inthe case of Acacia mearnsii tannin composed of the pyrogallolB-ring, amination in the presence of dioxygen produces4′-amino-3′,5′-dihydroxybenzene on the B-ring. In the case of thecatechol B-ring of quebracho tannins, an amination reactiondoes not take place either with or without O2.

Different amination conditions were tested.161,162 A NH3

solution containing tannins is stirred for one hour. Themixture becomes very viscous, and then placed for one dayunder hood at room temperature. The solid obtained iswashed with water and dried. Analysis showed that a multiami-nation took place on a majority of phenolic hydroxyls regard-less of the ring, A or B. On the other hand, an oligomerizationoccurred due to the formation of imine bonds between variousunits involving a tannin-ammonium gelation of the mixture(Fig. 31).

4.4.4. Reactions between tannin and epoxy groups4.4.4.1. Reactions with epoxy groups. Tannins can react

with various epoxies. Hydrolysable tannins were used to makeion-exchange resins after reaction with epoxy resin. Tannin OH

Fig. 27 Synthesis of tetra-O-ethyl-3,7,3’,4’-ethyl[3H]-5-quercetin.

Fig. 28 Methylation of chrysin.

Fig. 29 Synthesis of 7-O-methyl quercetin.

Fig. 30 Carbomethylation of pine tannin by chloroacetic acid.



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reacts with epoxy groups.163 Condensed tannins can also reactwith epoxy.164 This is particularly the case with pine tanninswith diglycidyl ether or polyglycidyl ether (Fig. 32).

4.4.4.2 Addition of epoxy groups. Currently, companieswant to substitute bisphenol-A because of its toxicity. Thiscompound used in epoxy-polyphenolic resins was substitutedby catechin. After reaction of catechin with epichloridrin(Fig. 33), the compound thus formed can then replace thebisphenol A diglycidyl ether usually used.165

4.4.5. Alkoxylation. Like other biobased resources such aslignin,166 tannins can be oxypropylated (Fig. 34). This reactionis a ring opening polymerization (ROP) of propylene oxide bythe hydroxyls of the polyphenols. To optimize the conditionsof synthesis, the reaction with resorcinol was used as a model.The best parameters of synthesis were applied to catechin,pine and quebracho tannins.159 The syntheses were in mass ina reactor with triethylamine (TEA) as a catalyst at 110 °C for

24 hours. The influence of the propylene oxide content wasinvestigated. With the increase in the load of propylene oxide,the percentage of weight gain of tannin during alkoxylationalso increased.

Oxypropylation was tested on pine barks at room tempera-ture in aqueous alkaline solution. Different degrees of substi-tution were obtained, always holding secondary hydroxylgroups at the end chain of poly(propylene oxide).167

Oxypropylation was also tested directly on gambier tanninswith propylene oxide and butylene oxide with potassiumhydroxide as a catalyst at 150 °C in a reactor.114,168 In bothcases, the reaction occurred and, by adjusting the reactionparameters, such as the oxide/tannin ratio, the length of poly-ether grafted chains can be controlled. Alkoxylation with buty-lene oxide offered an advantage to produce fully biobasedpolyols and was also successfully carried out with differenttannins such as gambier, mimosa or pine.168 For both oxides,whatever the tannin species, all OH present in moleculesreacted. Nowadays, an alkoxylation reaction is an encouragingway to increase the value of tannins, leading to reactivepolyols.

4.4.6. Reaction with isocyanates. To understand tanninreactivity with isocyanates to form urethane groups, catechinwas studied as a tannin model with phenyl isocyanate.169 Thereaction is performed with acetonitrile under an inert atmos-phere at 30 °C for 24 hours.

This study showed that OH from the A-ring does not reactwith isocyanate. It is quite surprising that only OH presentfrom the B-ring can take part in this reaction (Fig. 35).169 Thisis probably due to lower electron densities at the oxygen atomsin the A-ring compared to those in the B-ring. This suggeststhat only phenolic hydroxyl groups on the B-ring of a con-densed tannin molecule participated in its reaction withisocyanate.

Fig. 31 Example of structure obtained after amination of catechin byNH3.

Fig. 32 Reaction of pine tannins with ethylene glycol diglycidyl ether.

Fig. 33 Reaction of catechin glycidylation.

Fig. 34 Alkoxylation of catechin with propylene oxide.

Fig. 35 Reaction of catechin with phenyl isocyanate.



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5. Toward biobased polymers andmaterials

5.1. Adhesives

As mentioned, the main aldehyde to form adhesives is form-aldehyde.170 However, the resulting adhesive formaldehyde–tannins have a small number of methylene bonds, whichexplains the brittleness often observed. To improve theadhesive, phenol or resorcinol molecules can be added tocreate bonds between distant tannin sites through methylenelinks120,171 with a decrease of the viscosity of the mixture.Tannins can be used in adhesive particle board resins withanother natural component such as starch. A study has provedthe industrial viability of using cornstarch–mimosa tannin–urea formaldehyde resin in a classic adhesive formulation inorder to replace the conventional urea–formaldehydesystem.172 If cornstarch and mimosa tannin are added up to10 and 4%, respectively, the corresponding resins for boardsdo not present variations in their physical and mechanical pro-perties. Other additives can be used to improve the adhesiveproperties. A study was performed with glyoxal because it isnot toxic and produced at an industrial scale.173 Testswere carried out on wood panels which were glued with anadhesive formulated with tannin pine and glyoxal. Glyoxalreactivity with phenol, urea and melanin is lower than thereactivity of formaldehyde. This study also showed that cohe-sion between these panels was lower compared to those gluedwith a tannin-paraformaldehyde adhesive. However, in thecase of formaldehyde prohibition, the glyoxal can be a poten-tial substituent. Chestnut tannins were used for woodadhesives with formaldehyde, tris(hydroxymethyl)nitro-methane, and with glyoxal and hexamine as hardeners.126 Thisstudy was performed in order to substitute formaldehyde. Thecuring enthalpy for the adhesive with hexamine as a hardenerwas the highest, due to a high crosslinking density. But thehighest rate of chemical curing was achieved using paraformal-dehyde as a hardener to achieve complete chemical curing.Another option to synthesize adhesives without formaldehydeis with hexamine. For mimosa tannins, good adhesives forwood panels can be obtained at pH = 10. Composite materialswith good properties have been elaborated starting from non-woven fiber of hemp and flax fibers with a tannin mimosaresin containing 5% of hexamine.133 Thus, it is possible tomake multi-layered compounds by binding fibers togetherwith the resin.174 These composites made with tannin resinreinforced with flax fibers can be used for automotiveapplications.175

Although formaldehyde gives good adhesive properties, thecorresponding volatile organic compounds (VOC) bring sometoxicity issues. Then, some studies were carried out with newadhesives starting from acetaldehyde, propionaldehyde, n-butyraldehyde or furfural. These aldehydes were formulatedwith mimosa tannins as adhesives for wood panels176 whichshowed poor adhesion properties. However, partial substi-tution of formaldehyde by n-butyraldehyde can be carried out.

The latter is water resistant due to the hydrophobicity ofthe carbon chain of n-butyraldehyde. To increase theproperties, in addition to tannins and aldehydes, the formu-lation can contain urea such as monomethylol or dimethylolurea.177 Another substitute for formaldehyde which wasrecently studied and which gives satisfactory adhesion resultsis furfuryl alcohol,178 which is often used for tannin-basedfoams.

Acylation is also a useful chemical modification to decreasethe brittleness of materials based on tannins and form-aldehydes. Adipoyl dichloride is used to introduce a carbon-aceous chain between tannin molecules. This inter-chainbrings more flexible materials.179

5.2. Phenol-formaldehyde foam type

As phenol-formaldehyde foams, it is also possible to syn-thesize porous structures from tannins. A customary foam for-mulation is the following: tannins, furfuryl alcohol (used as anexothermic agent) and formaldehyde. Foams were first elabo-rated with catechin under acidic conditions.134 The first foamsbased on tannins were synthesized with mimosa tannin andcatalyzed by p-toluene sulphonic acid.180 Since then, severalstudies have been reported to analyze different parameterssuch as acid and alkaline catalysis,181 the use of boric or phos-phoric acid to increase fire resistance,182 the influence of foamcarbonization,183 the effect of the pre-activation of tannins,184

the addition of hydroxymethylated lignin, polyurethane or anindustrial surfactant.136 The processing conditions have alsobeen investigated. Microwaves were e.g. tested to accelerate thereaction and led to foams which have comparable propertiesto those obtained under a hot-press procedure.185 Thus, micro-wave-based systems can be considered as more appropriateprocesses for industrial applications because of the corres-ponding reduced production times. Different extracts oftannins were tested.136 In the case of tannins with a lowdegree of polymerization, it is not possible to obtain foams. Itis in particular the case of the gambier tannin which containstoo great a proportion of monoflavonoid.186 The reactivityvaries according to the nature of tannin. For instance, pinetannins are very reactive. This strong reactivity involves ashorter time of freezing which does not leave time for thefoaming agents to evaporate. Only recently a formulation wassuccessfully developed based on propylene glycol.187

Glyoxal was also used for foam applications as a form-aldehyde substituent to prepare tannin/furanic rigid foams.188

Glyoxal can completely substitute formaldehyde in pine foams.The corresponding insulation materials have low thermal con-ductivity and good mechanical resistance.189 As previouslymentioned, furfuryl alcohol can react with tannins to givetannin-furanic foams. This study investigates the integrationof tannins into industrial processes to obtain light porousmaterials with good properties. Tannin open-cell foams hadgood sound absorption/acoustic insulation characteristics,compared to industrial foams.

A new foam structure can be prepared from an emulsionbased on tannin, hexamine, vegetable oil and a surfactant.



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After the reaction of the resin, oil is extracted and the remain-ing structure is pyrolysed.190 These foams have more openstructures with smaller pores and higher mechanical pro-perties, compared to tannin-formaldehyde carbon foams.191 Astudy shows that the initial concentration of tannins controlsthe porosity, the average cell size and the cell wall thickness ofthe foam.192

With the previous formulations, the tannin foams areeither rigid or semi-rigid. Tests were carried out to synthesizeflexible foams. During the preparation, glycerol is added withtannins, furfuryl alcohol and formaldehyde.193 Glycerol acts asa non-volatile and non-toxic plasticizer.

5.3. Materials based on polyurethane

5.3.1. Polyurethane foams. Polyurethane foams wereobtained starting from mimosa tannins and diisocyanates,such as the methylene diphenyl diisocyanate (MDI).194 Toincrease the reactivity, tannins are initially treated by a lique-faction process based on polyethylene glycol and glycerol.Liquefaction takes place at 120 °C for 1 h. Then, the catalystand other additives are mixed with tannin derivatives beforethe addition of MDI. In another study, condensed tannins areused to make flexible and open cell polyurethane foams. Queb-racho was tested with ethoxylated fatty amine and polymericMDI.195 Foams are composed of copolymerized amine/isocya-nate/tannin oligomers. From 30 up to 50% of tannins can beadded with the reticulating agents. These foams slow downburning compared to conventional polyurethane foams.

Closed-cell foams were also obtained from oxypropylatedtannins. Oxypropylated glycerol was replaced progressively upto 100%. The use of tannins increased the properties, e.g. thecompressive strength and closed-cell content, leading to areduction of the thermal conductivity.

5.3.2. Non-porous polyurethane materials. It is also poss-ible to form polyurethane films with partially benzoylatedmimosa tannins (Fig. 36).121 Benzoylation is performed via theSchotten–Baumann method (section 4.4.1, Fig. 24). Bifunc-tional isocyanates are then used to elaborate reticulated films,which are bright with strong scratch resistance.

Crosslinked polyurethane membranes were also preparedfrom oxypropylated gambier tannins by a two-step procedurewith isocyanate-terminated prepolymers. These were preparedfrom MDI with poly(propylene)glycol (PPG) or with a polyesterdiol from fatty acid dimers. Oxypropylated gambier tanninsoffer the potential of preparing a wide range of products. The

nature of isocyanate-terminated prepolymers and the isocya-nate/hydroxyl group molar ratio impacted the behaviour (ther-moplastic vs. thermoset), the morphology (different crosslinkdensities) and the properties of the final polyurethanematerials. For example, a high elongation at break with a lowmodulus is obtained for membranes with the longest graftedPPG chains.

5.4. Materials based on polyesters

As previously mentioned, tannins can be modified with fattyacids and it is possible to obtain fully biobased polyesterthermosets. Vegetable oil was used afterwards as a crosslinkingagent to obtain films.148 Tannins were esterified with linoleicacid and then crosslinked. The oxidation copolymerizationwas promoted with cobalt/zirconium metal driers as the cata-lyst. Tannin derivatives provided rigidity through polyphenolicaromatic rings and unsaturated chains as crosslinkers. Asimilar study was carried out with oleic acid and provided soft,flexible rubber-like materials.196

5.5. Materials based on epoxy resins

Hydrolysable tannins can also act as a crosslinking agent forepoxy resins. The corresponding resins have great resistanceproperties while being easily decomposable for the recy-cling.197 These resins can be dispersed in solvent and used asa coating to protect metals from corrosion.198 Condensedtannins can also react with di-epoxy to form resins.164 Resinswere synthesized with pine tannins with diglycidyl ether orpolyglycidyl ether. The effect of the pH has been explored and,e.g., it was shown that epoxy opening is favored under basicconditions with an increase of the reaction kinetics. Recently(2014), a new project called “Green Epoxy”, from Bpifrance andFUI (France), was recently launched to develop a non-toxicindustrial alternative to rigid epoxy resins derived from the for-estry industry.

As previously mentioned (section 4.4.4.2), tannins can bemodified by epichloridrin and then formulated with amines oranhydrides as reticulating agents.199 Resins based on green teaextracts were functionalized with epichlorohydrin in the pres-ence of benzyltriethylammonium chloride as a phase transfercatalyst.200 To obtain epoxy resins, tannin epoxy derivativeswere cured with isophorone diamine. Thermal and mechanicalproperties of tannin based epoxy resins are high due to a highcrosslinking density.

6. Conclusion

Tannins are an abundant natural aromatic resource that hasnot yet been fully exploited as a renewable source to elaboratenew macromolecular architectures for novel materials. Thespecific chemical structures of tannins allow bimolecularnucleophilic substitution, giving the opportunity to convertthem into several promising aromatic chemicals and buildingblocks. Neat or modified tannins permit the elaboration ofadhesives, foams, polyurethanes, epoxy resins etc. with inter-

Fig. 36 Reaction between partially benzoylated mimosa tannins andisocyanates.



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esting performance. However, the intrinsic properties oftannins, the resource variability, polymerization degree andchemical hyperbranched structures have hampered the develop-ment of tannin derivatives. Thus, to obtain high valuetannin products, a preliminary investigation into the chemicalstructure of tannins as well as the use of high purity tanninsare a priority.

The tannin price varies between 0.7 and 1.5 € kg−1, accord-ing to the extraction, purification and drying process anddepending on the botanical resource and the product purity.Such a price can be a limitation for some low-cost applicationscompared to some other renewable resources.

In summary, all these current developments highlight thegreat potential of tannins to create innovative and efficientmaterials in agreement with the emergent concept of sustain-able development. Tannins as an aromatic building block forgreen chemistry to develop biobased polymers represent animportant field of research. Tannins have become an encoura-ging and interesting renewable aromatic resource for the nextfew decades.

Acknowledgements

The authors thank Professor A. Pizzi and J.-P. Pascault forsome helpful discussions. We are grateful to Alsace Region,Oseo (Bpifrance) and ANRT for financial support.

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Chemical modification of tannins to elaborate aromatic